The invention relates to a cooling system with an electrically adjustable control element for influencing the coolant temperature of an internal combustion engine in a motor vehicle.
A cooling system of this generic type is disclosed, for example, in German patent document DE 43 24 178 A, and corresponding allowed U.S. patent application Ser. No. 08/277,004. This system incorporates a radiator and a thermostatic valve in the form of an electrically adjustable control element, by which the coolant temperature can be regulated during warmup operation, mixed operation, and radiator operation. The electrically adjustable thermostatic valve contains an element composed of expandable material that is electrically heatable to reduce the coolant temperature.
In this known cooling system, the thermostatic valve regulates the flow of coolant between the engine and the radiator so that during warmup operation, the coolant coming from the engine returns through a short circuit, bypassing the radiator. During mixed operation, the coolant coming from the engine returns partly through the radiator and partly through the short circuit, and during radiator operation the coolant coming from the engine returns primarily through the radiator. Electrically heating the electrically adjustable control element enlarges the aperture cross section, allowing more coolant to flow to the radiator compared to an aperture cross section governed by the temperature of the coolant.
Electrical heating of the electrically adjustable control element is performed by a regulating device that detects the actual coolant temperature and compares it with a preset coolant temperature. If the actual coolant temperature detected is above the set coolant temperature, the electrical heating is switched on to cool the coolant. On the other hand, at an actual coolant temperature below the prescribed set coolant temperature the electrical heating of the electrically adjustable control element is switched off.
This known cooling system performs only two-point control by means of a simple comparison of the actual coolant temperature and the set cooling temperature, so that severe underswings and overswings can occur with respect to the set coolant temperature.
Moreover, a cooling system disclosed in German patent document DE 44 03 713 (not published) has an electrically adjustable delivery pump which serves as a control element for influencing the coolant temperature. However, no control strategy is described for this device.
The object of the invention is to provide an improved cooling system of the type described above, which prevents underswings and/or overswings relative to the preset coolant temperature; and reaches the preset coolant temperature as quickly as possible.
This object is achieved by the cooling system according to the invention in which a known PID (Proportional-Integral-Derivative) feedback control is used to regulate the electrically adjustable thermostatic valve to achieve optimum control of the coolant temperature. The parameters for the regulating device are determined adaptively during engine operation by evaluating the response of a value proportional to the actual coolant temperature, to a step excitation input to the control element.
It has been found in tests that a PID regulator is especially suitable for regulating a cooling system with an electronically or electrically adjustable control element to regulate the coolant temperature of internal combustion engines so as to reach the preset coolant temperature as quickly as possible. A PID regulator of this kind is well known to those skilled in the art, and can be of either analog or digital design. It can be integrated for example into an electronic control device, (which is provided in any case), for controlling the cooling system and/or the engine. Particularly when using a digital PID regulator, the costs and effort for a cooling system according to the invention are slight, despite improved regulation. The use of PID control units for feedback control applications has long been known to those skilled in the field of control engineering. However, the regulator can also be another type of known electronic regulator, for example a PInDn, or a PI regulator.
It should also be noted that the parameter influenced by the coolant temperature (or the value that is proportional to the actual coolant temperature) can also be the (actual) coolant temperature itself.
The regulator according to the invention simulates the relationship between the input of the control element and the output of the cooling system. The parameters of the regulator are automatically adjusted to take account of all the parameters that affect in this relationship, such as vehicle speed, outside temperature, engine load, vehicle interior heating, the state of the air conditioning system, the rpm of the radiator fan, and/or temperature acquisition subject to dead time as well as the inertia and nonlinearity of the control element. Additional sensors to detect these parameters are thus not required, so that not only optimum regulation, but also additional cost savings are achieved.
In one advantageous embodiment of the invention, step excitation is performed by the control signal value to control the control element itself. Assuming that the control signal value contains at least a partial step signal component, according to the invention a self-test signal does not have to be generated as a step excitation to determine the step response. Preferably the step response is evaluated when a predetermined operating condition prevails.
In another advantageous embodiment of the invention, the control signal value is pulse-width-modulated. Pulse-width-modulated signals permit finely tuned control of the control signal, and frequently a step excitation is automatically available to evaluate a step response.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.